Desulfurization catalysts used for oil refining, etc., and boiler sludge, boiler ash, etc. generated from petroleum fuels in thermal power plants, etc. contain high concentrations of valuable metals such as Mo, Ni, V, etc. Because these valuable metals are rare and extremely expensive, it is desirable to recover them in the form of high-concentration iron-based alloys, so that the wastes can be reused as resources.
Japanese Patent 3705472 discloses a method of heating a waste containing Ni, Mo and V at 450-950° C. to remove 1, N and C; mixing it with an iron source such as scale, etc. and a reducing agent such as coke, etc.; pulverizing and granulating the mixture; heating the resultant granules at 1150-1350° C. to conduct the solid-phase reduction of oxides of Fe, Ni and Mo; melting them to form a melt mainly containing Fe, Ni and Mo and a V-oxide-rich flux; subjecting the melt to a dephosphorization treatment to form a low-phosphorus (Ni, Mo)—Fe alloy; and mixing the flux with iron and the reducing agent and heating them to reduce the V oxide in the flux, thereby forming an Fe—V alloy. Because the separation of Ni and Mo from V is conducted by a solid-phase reduction method for reducing the oxides of Ni and Mo without reducing the V oxide in this method, a mixture of the waste, the iron source and the reducing agent is heated at relatively low temperatures of 1150-1350° C.
The above solid-phase reduction method, however, does not fully separate Ni and Mo from V, resulting in an Fe—Ni—Mo alloy containing a relatively large amount of V, and an Fe—V alloy containing relatively large amounts of Ni and Mo. Also, when a waste containing a large amount of P is used, V and P are not fully separated, resulting in a large amount of P contained in the Fe—V alloy. In an attempt to remove only P from this Fe—V alloy melt, V would be predominantly oxidized, resulting in extremely large loss of V by oxidation. Thus, if P were dissolved in the Fe—V alloy, it would actually be difficult to remove P from the Fe—V alloy. In addition, because the oxides of Fe, Ni and Mo are solid-phase-reduced with a carbonaceous reducing agent such as finely pulverized charcoal or coke, etc., the resultant Fe—V alloy contains C, and an attempt to remove C by oxidation like P would oxidize V simultaneously.
Japanese Patent 3450779 discloses a method for recovering metal components from a used catalyst containing V, Mo, Ni and Co with an Al2O3 carrier, comprising the steps of (a) roasting the used catalyst at 500-800° C. to oxidize the metal components, (b) adding Si and/or Al in an amount of 50-120% by mass based on the stoichiometric amount for reducing the oxides of Mo, Ni and Co to metals, together with CaO, (c) heat-reducing them to form a Mo—Ni—Co alloy and a CaO—Al2O3 slag containing V oxide, (d) adding Si and/or Al in such an amount as to sufficiently reduce the V oxide contained in the slag separated from the Mo—Ni—Co alloy, (e) forming a V—Si alloy or a V—Al alloy and a CaO—Al2O3 slag by heat reducing, and (f) separating the V—Si alloy or the V—Al alloy from the slag.
It is actually difficult, however, to control the amount of a reducing agent (Si and/or Al) to reduce only the oxides of Mo, Ni and Co without reducing the V oxide. For instance, when too much Si and/or Al are added, a large amount of the V oxide is reduced, and V is dissolved into the Mo—Ni—Co alloy. On the other hand, when too small amounts of Si and/or Al are added, part of the oxides of Mo, Ni and Co are not oxidized, remaining in the V-oxide-containing slag. Because there is no uniform reducing environment in an actual operation, Si and/or Al may be partially oxidized before participating in the reduction reaction, even if they are weighed accurately. Thus, the amounts of Si and/or Al acting as reducing agents would become insufficient, causing the above problems. It is clear from above that the method of Japanese Patent 3450779 actually fails to separate V from Mo, Ni and Co sufficiently.
Recently, desulfurization catalysts used for oil refining contain an increasingly larger amount of P to exhibit higher performance, but it is difficult to remove only P from a P-containing V alloy because P and V have close oxidizability. Accordingly, when the method of Japanese Patent 3450779 is conducted on the used desulfurization catalysts containing a large amount of P, the formed V—Si alloy or V—Al alloy contains too much P to reduce its concentration, for instance, to a level corresponding to the ISO FeV40 standard, by which the concentration of P is 0.1% by mass or less in an iron-based alloy containing 35-50% by mass of V.
Japanese Patent 3705498 discloses a method for recovering valuable metals of V, Mo and Ni from a waste containing the valuable metals, comprising the steps of (a) roasting the waste at 800-950° C. to form the oxides of V, Mo and Ni, (b) reducing the oxides of Mo and Ni in the waste with Fe to form an Fe—Mo—Ni alloy and a V-oxide-containing slag, and (c) adding a reducing agent to the V-oxide-containing slag to form an Fe—V alloy. However, like Japanese Patent 3450779, weak reduction with Fe fails to separate Mo and Ni from V sufficiently, resulting in large amounts of Mo and Ni contained in the V-oxide-containing slag. Further, when this method is conducted on used desulfurization catalysts with much P, a considerable amount of P remains in the V-oxide-containing slag, resulting in a high concentration of P in the Fe—V alloy formed in a subsequent step, which causes the deterioration of quality.